Activation of Persulfates by Graphitized ... - ACS Publications

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Activation of Persulfates by Graphitized Nanodiamonds for Removal of Organic Compounds Hongshin Lee,† Hyoung-il Kim,† Seunghyun Weon,‡ Wonyong Choi,‡ Yu Sik Hwang,§,∥ Jiwon Seo,⊥ Changha Lee,*,⊥ and Jae-Hong Kim*,† †

Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-dong, Pohang 790-784, Republic of Korea § Future Environmental Research Center, Korea Institute of Toxicology (KIT), Jinju, 660-844, Republic of Korea ∥ Human and Environmental Toxicology Program, University of Science and Technology (UST), Daejeon, 305-350, Republic of Korea ⊥ School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulju-gun, Ulsan 689-798, Republic of Korea ‡

S Supporting Information *

ABSTRACT: This study introduces graphited nanodiamond (G-ND) as an environmentally friendly, easy-to-regenerate, and cost-effective alternative catalyst to activate persulfate (i.e., peroxymonosulfate (PMS) and peroxydisulfate (PDS)) and oxidize organic compounds in water. The G-ND was found to be superior for persulfate activation to other benchmark carbon materials such as graphite, graphene, fullerene, and carbon nanotubes. The G-ND/persulfate showed selective reactivity toward phenolic compounds and some pharmaceuticals, and the degradation kinetics were not inhibited by the presence of oxidant scavengers and natural organic matter. These results indicate that radical intermediates such as sulfate radical anion and hydroxyl radical are not majorly responsible for this persulfate-driven oxidation of organic compounds. The findings from linear sweep voltammetry, thermogravimetric analysis, Fourier transform infrared spectroscopy, and electron paramagnetic resonance spectroscopy analyses suggest that the both persulfate and phenol effectively bind to G-ND surface and are likely to form charge transfer complex, in which G-ND plays a critical role in mediating facile electron transfer from phenol to persulfate.

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INTRODUCTION

Several strategies have been developed to activate persulfates into radical species, which include heating,9−13 UV irradiation,14 adding bases,8 and catalysis with transition metals.3,15 Among them, the catalytic activation by transition metals is considered the most simple and sustainable method without requiring external energy or consumable chemicals. In general, the metal-catalyzed activation of persulfates is believed to proceed via the mechanism analogous to that of Fenton-like reaction, where persulfates are decomposed into radical species through the redox cycle involving reduced/oxidized metal ions (Reactions 1 and 2). Ionic iron, cobalt, copper, manganese, silver, and titanium have been found to catalyze the persulfate decomposition.2,3

Persulfates such as peroxydisulfate (PDS) and peroxymonosulfate (PMS) have been extensively studied as an alternative oxidant for degrading aqueous organic contaminants during the past decade.1−4 Persulfates themselves are strong oxidants that favor two-electron transfer reactions (E0(S2O82−/SO42−) = 1.96 VNHE for PDS5 and E0(HSO5−/SO42−) = 1.75 VNHE for PMS6). However, in most past studies, persulfates were used in combination with catalysts or externally supplied energy to cleave the peroxide bond and generate more reactive radical species such as sulfate radical anion (SO4•−, E0(SO4•−/SO42) = 2.43 VNHE)7 and hydroxyl radical (•OH, E0(•OH/H2O) = 2.81 VNHE5). These activated persulfate systems have been suggested as an effective approach to treat a broad spectrum of refractory organic contaminants in various environmental remediation scenarios (e.g., in situ chemical oxidation for groundwater remediation and advanced oxidation process in wastewater treatment).8,9 © XXXX American Chemical Society

Received: April 26, 2016 Revised: August 15, 2016 Accepted: August 19, 2016

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DOI: 10.1021/acs.est.6b02079 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

phenol, aniline, bisphenol A, acetaminophen, carbamazepine, sulfamethoxazole, ranitidine, benzoic acid (BA), methanol, tertbutanol, dimethyl sulfoxide, L-histidine, sodium azide, phosphoric acid, perchloric acid were purchased from Sigma-Aldrich Co. All solutions were prepared using deionized water from a Milli-Q ultrapure water-purification system (>18.2 MΩ·cm, Millipore Co.). Stock solutions of persulfates (100 mM) and organic compounds (1 mM) were prepared and stored at 4 °C prior to use. Preparation and Characterization of NDs. Bare NDs in powder form were prepared by suspending as-received ND in water (ca. 5 wt%), rinsing with 1 M sulfuric acid solution, filtering, washing with deionized water, and drying. Oxidized NDs (O-NDs) were prepared by annealing bare NDs at 430 °C for 5 h in a muffle furnace under ambient atmosphere.25 Graphitized NDs (G-NDs) were prepared by annealing NDs at 600−1200 °C for 2 h under Ar atmosphere.26 The morphology and surface properties of NDs were characterized via a highresolution transmission electron microscopy coupled with an energy dispersive X-ray spectrometer (HRTEM/EDS at 200 kV, JEM-2100F, JEOL Ltd.), an X-ray photoelectron spectroscopy (XPS using Al Kα lines, K-Alpha, Thermo Scientific Co.), a Raman spectroscopy (Raman, Alpha300S, WITec. Co.), thermogravimetric analysis (TGA, Q600, TA Instrument Co.; under the N2 flow condition), FT-IR spectroscopy (FT-IR, Agilent 670, Agilent Co.), and Brunauer-emmett-teller surface area analysis (BET, ASAP 2420, Micromeritics Instruments Co.). Batch Experimental Procedure. Batch experiments were conducted for suspensions containing carbon nanomaterial, target organic compound, and phosphate buffer (1 mM, pH 7) in 50 mL vials at room temperature (20 ± 2 °C). The solution pH varied by less than 0.2 units during the reaction. Phosphate did not appear to affect the reaction; no difference in the phenol removal was observed in unbuffered solution (Supporting Information (SI) Figure S1). The solution was in equilibrium with atmosphere in terms of oxygen and the control experiment confirmed that the oxygen did not affect the phenol removal in G-ND/PDS system (SI Figure S2). The reaction was initiated by adding an aliquot of persulfate stock solution. Samples were withdrawn by a 1 mL syringe and immediately filtered using a 0.45-μm PTFE syringe filter to remove the carbon material and terminate reactions. For select experiments, methanol, BA, tert-butanol, dimethyl sulfoxide, Lhistidine and azide ions were used as probe compounds or scavengers for reactive oxidants. Most experiments were conducted in duplicates or triplicates, and the average values with standard deviations were presented. Additional set of experiments was performed using natural water catchments (surface, ground, and sea waters) after filtering with a 0.45 μm membrane. The water quality parameters of these water samples are summarized in SI Table S1. Analytical Methods. The concentration of organic compounds and persulfate was analyzed using a high-performance liquid chromatography (HPLC, Agilent 1260 infinity, Agilent Co.) equipped with an UV/vis detector (230 nm for PDS, 277 nm for phenol, 229 nm for aniline, 315 nm for ranitidine, 230 nm for bisphenol A, 241 nm for acetaminophen, 277 nm for sulfamethoxazole, 285 nm for carbamazepine, and 227 nm for BA). Separation was performed on a ZORBAX Eclipse XDB-C18 column (250 mm × 4.6 mm, 5 μm) using a binary mixture as mobile phase (0.1% (v/v) aqueous phosphoric acid and neat acetonitrile) at a 1.0 mL/min flow

Mn + + S2 O82 −(or HSO5−) → M(n + 1) + + SO4•− + SO4 2 −(or SO4•− + OH−)

(1)

M(n + 1) + + S2 O82 −(or HSO5−) → Mn + + S2 O8•−(or SO5•− + H+)

(2)

Alternatively, a recent study proposed that PDS can be also activated by metal oxides such as copper oxide (CuO) through a nonradical mechanism, in which the outer-sphere interaction between the CuO surface and PDS causes the electron rearrangement in the PDS molecule and subsequently increases the oxidizing power of PDS.15 Carbonaceous materials also have been proposed as a new class of nonmetal persulfate activator in a few recent studies.16−19 Despite the prevailing evidence that a coupling of persulfates with select carbon materials produces reactive species that effectively degrades organic compounds, there exist contradicting interpretations with respect to the identity of the reactive species and the activation mechanisms involved (i.e., radical vs nonradical mechanisms). These studies mostly focused on carbon nanotubes (CNTs), reduced graphene oxide (rGO), mesoporous carbon (CMK-8), and biochar as a persulfate activation catalyst. Another class of carbon nanomaterial that can be potentially suited for persulfate activation is nanodiamond (ND). ND consists of nanosized sp3 diamond core (where the name originates from) surrounded by sp2 carbon layer on the surface. ND has been increasingly being explored as a superior and relatively inexpensive alternative carbon nanomaterial in various applications such as sensors,20 biomedicine,21 and bioimaging22 due to unique electrical and photoluminescent properties that can be tailored through facile surface functionalization as well as core element doping. A relatively mediocre performance of pristine ND for persulfate activation compared to CNT and rGO has been reported,23 leaving other surface functionalized NDs as a largely untapped opportunity for this application. We herein report that a surface-modified ND, particularly through graphitization, can effectively activate persulfates for the removal of select organic compounds at higher efficiencies compared to benchmark carbon materials such as CNTs and rGO. We further discuss a unique mechanism involved with persulfate activation by ND, which contrast to a mechanism proposed by Duan et al.24 who recently reported the enhanced persulfate activation by similarly graphitized NDs, based on the results from linear sweep voltammetry, thermogravimetric analysis (TGA), and Fourier transform infrared (FT-IR) spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy.

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MATERIALS AND METHODS Reagents. All chemicals were of high-purity reagent grade and used without further purification. Chemicals used in this study include ND (uDiamond Allegro solution, Carbodeon Co.), multiwalled carbon nanotubes (MWCNT, Comocat Co.), single-walled carbon nanotubes (SWCNT, Nanolab. Co.), fullerene (C60, SES Research), Suwannee River natural organic matter (SR-NOM), Suwannee River humic acid (SR-HA), Suwannee River fulvic acid (SR-FA), Elliott Soil humic acid (ES-HA) (International Humic Substances Society, IHSS), acetonitrile (J.T. Baker Co.), and sodium hydroxide solution (Fluka Co.). Potassium PDS, potassium PMS, sodium periodate (PI), graphite, graphene oxide, hydrogen peroxide, B


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Figure 1. (a)−(c) Photograph and (d)−(e) HR-TEM images of different ND samples: (a) and (d): bare ND; (b) and (e): O-ND; (c) and (f): GND.

Figure 2. (a) Raman, (b) FTIR, and (c) XPS spectra of different ND samples.

potential was swept from 0.3 to 1.0 V (vs Ag/AgCl) several times. Chronoamperometries were carried out under the same condition as the LSV but with the working electrode biased at an applied potential of +0.8 V (vs Ag/AgCl), and electrochemical measurements were subsequently added into the PEC reactor at stated intervals with final concentrations of 1 mM PDS and 0.1 mM phenol, respectively.

rate. The detailed experimental setup and procedure information on GC/MS system for phenol oxidation products analysis is summarized in SI Text S1. The occurrence of radical species such as SO4•− and •OH was examined with an electron paramagnetic resonance (EPR) spectroscopy (ELEXYS E580, Bruker Co.) using 10 mM 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin-trapping agent. The sample was placed in a quartz flat cell and analyzed under the following conditions: microwave frequency = 9.64 GHz, microwave power = 0.94 mW, modulation frequency = 100 kHz, and modulation amplitude = 2.0 G. Linear Sweep Voltammetry (LSV) and Chronoamperometry. G-NDs and O-NDs (5 mg) were first dispersed in 29 μL of nafion perfluorinated resin solution (5 wt %, Aldrich) and 400 mL 2-propanol (99.9%, Aldrich). The mixture (5 μL) was dropped onto a glassy carbon electrode and dried for 10 min at 105 °C; this procedure was repeated three times. All electrochemical measurements were performed using a computer-controlled potentiostat (Gamry, Reference 600). The reactor contained a glassy carbon electrode, a coiled Pt wire, and a Ag/AgCl/KCl (sat) electrode as a working, counter, and reference electrode, respectively and 0.1 M phosphate buffer (pH ≈ 7) as an electrolyte. The current at a working electrode of the linear sweep voltammetry was measured as the

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RESULTS AND DISCUSSION Properties of NDs. Photographs in Figure 1a−c show that bare ND and O-ND exhibit gray color, whereas G-ND acquires distinct black color through graphitization, with higher temperature annealing resulting in a darker color (SI Figure S3a). HR-TEM images also suggest that surface modification leads to changes in the morphology of NDs (Figure 1d−f, SI Figure S4). Bare NDs appear as irregular agglomerates of primary nanoparticles with sizes of 4−6 nm, whereas O-NDs show more dense agglomerates of slightly shrunk primary particles (ca. 1−3 nm). G-ND particles were in similar size to bare ND but exhibited unique onion-like structures due to formation of multiple graphitic carbon layers around the perimeter, which are presumably responsible for the apparent color change.25 While the particle agglomeration observed in

C


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Figure 3. Phenol removal by (a) surface-modified NDs and (b) G-ND annealed at different temperatures in the presence of PDS, (c) removal of various organic compounds by G-NDs in the presence of PDS: [phenol]0 = [aniline]0 = [ranitidine]0 = [bisphenol-A]0 = [acetaminophen]0 = [sulfamethoxazole]0 = [carbamazepine]0 = [benzoic acid]0 = 0.01 mM; [ND]0 = [O-ND]0 = [G-ND]0 = 0.1 g L−1; [PDS]0 = 1 mM; 1 mM phosphate buffer at pH0 7.

Figure 4. Phenol removal by carbon-materials (a) without and (b) with PDS, (c) decomposition of PDS by carbon-materials without and with PDS: [phenol]0 = 0.01 mM; [C60]0 = [graphite]0 = [graphite oxide]0 = [GO]0 = [rGO]0 = [MWCNT]0 = [SWCNT]0 = [G-ND]0 = 0.1 g L−1; [PDS]0 = 1 mM; 1 mM phosphate buffer at pH0 7.

(1728−1757 cm−1), C−H (2853−2962 cm−1), and O−H stretching (3280−3675 cm−1).25,30,31 Presence of oxygen functionality in bare-ND is also evident by O 1s peak at 530 eV in XPS spectrum (Figure 2c). A similar pattern in FT-IR and XPS spectrum was observed in O-ND with overall slightly higher peak intensity, due to the surface oxidation. In particular, the increase in a prominent IR peak due to the CO stretching vibration (1790 cm−1) was observed. Meanwhile, GNDs does not exhibit significant IR-absorption nor XPS O 1s peaks, indicating the loss of oxygen functionality. When the annealing temperature was increased, the O 1s signal was further decreased (SI Figure S3b). The XPS analysis also confirms that ND samples used in this study do not contain transition metal impurities such as iron or copper at a quantifiable level. Removal of Organic Compounds by Persulfates in Combination with NDs. Phenol was used as a representative organic pollutant in our study. PDS alone did not oxidize phenol (results not shown). The phenol removal by NDs in the absence of PDS was also negligible, indicating that the phenol removal by adsorption onto the ND surface is also insignificant at the loading of ND used in this study (SI Figure S6). When PDS and G-ND were added at the same time, we observed complete phenol removal within 10 min. In contrast, the bare

TEM could have partly resulted from TEM sample drying, DLS analysis results (SI Figure S5) confirm that G-NDs are also present in the aqueous phase as agglomerates with average size of 185 ± 10 nm. The Raman spectrum of bare ND (Figure 2a) exhibits characteristic diamond (∼1323 cm−1),27,28 D-band (∼1410 cm−1),29 and G-band peaks (∼1587 cm−1),29 consistent with the past reports. The ratio of diamond peak intensity to G-band peak intensity, I(diamond)/I(G‑band), was relatively high at 2.08, which indicates dominant presence of sp3 carbons (in core) and partial presence of graphitic sp2 bonds (in shell) in the bare ND.25 After oxidation (O-ND), the ratio further increased to 2.97 (i.e., more sp3 carbon) due to oxidation of graphitic sp2 carbons at the surface to oxygen-containing sp3 carbons or potential loss of some sp2 carbon through oxidation to carbon dioxide. In contrast, the intensity ratio of I(diamond)/I(G‑band) decreased to 0.99 for G-ND, which resulted from the anticipated conversion of sp3 into sp2 bonds through graphitization. Such a graphitization might not be limited to the surface but have extended into the core structure, forming multiple onion-like graphitic layers (Figure 1f). Consistently, the FT-IR spectrum of G-ND showed clearly distinctive patterns from those of bare and O-NDs (Figure 2b). BareND exhibits multiple IR-absorption peaks credited to CC skeletal or O−H bending (1620−1660 cm−1), CO stretching D


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Environmental Science & Technology ND with PDS removed only 25% of phenol in 60 min and the O-ND with PDS did not remove phenol at all (Figure 3a). These observations and the fact that G-ND prepared at higher annealing temperature exhibited greater phenol removal in the presence of PDS (Figure 3b) collectively indicate that the graphitic sp2 carbons are responsible for the PDS activation. In other words, NDs with lower oxygen content (G-ND annealed at high temperature < G-ND annealed at lower temperature < bare-ND < O-ND) exhibit a greater catalytic activity. The rate of phenol removal in the G-ND/PDS system increased with increasing G-ND dose and decreasing initial phenol concentration (SI Figure S7). G-ND was found to also readily activate PMS and PI, resulting in effective remove phenol (SI Figure S8). In addition, the G-NDs/PDS system degraded various other organic compounds (Figure 3c) − the same set of organic compounds used in our previous study18 − but with widely varying efficiencies. Compounds such as acetaminophen, aniline, bisphenol A, sulfamethoxazole showed relatively higher degradation efficacies, whereas benzoic acid was somewhat resistant to degradation. The selectivity of this oxidation system on organic compound degradation suggests that •OH and SO4•− (i.e., nonselective reactants) are not likely involved. For example, benzoic acid, a compound that was hardly degraded in this system, has a high reactivity with both •OH and SO4•− (k = ∼ 109 M−1 s−1).32,33 We further compared the phenol removal performance of GND with different benchmark carbon materials such as C60, graphite, graphite oxide, GO, rGO, MWCNT, and SWCNT (Figure 4). In the absence of PDS (Figure 4a), phenol was found to adsorb to SWCNT and rGO but not to other carbons. In the presence of PDS (Figure 4b), phenol removal was the fastest with G-ND (the pseudo first order rate constants, k = 0.846 ± 0.132 min−1), followed by SWCNT (k = 0.260 ± 0.011 min−1) and MWCNT (k = 0.259 ± 0.013 min−1). The phenol degradation rates normalized by surface area (SI Figure S9) were found to be in the order of SWCNT (5.2 min−1 g/cm2) < MWCNT (11.0 min−1 g/cm2) < G-NDs (30.2 min−1 g/cm2). Other carbon materials such as rGO, graphite, graphite oxide, GO, and C60 showed very slow or no phenol degradation. Note that the phenol removal by rGO is mainly attributed to the adsorption, since the removal kinetics are similar to and without PDS (Figure 4a and b). The decomposition of PDS was also greater with G-NDs and CNTs (Figure 4c). A notable observation is that the PDS decomposition by G-NDs and CNTs was enhanced by phenol, suggesting that phenol is likely to serve as an electron donor to accelerate the activation of PDS by these carbon materials. No noticeable reaction between PDS and phenol was observed without carbon materials under the conditions employed in this study, although phenolic compounds are known to react with a high concentration of PDS under alkaline pH conditions.34 Reusability of G-NDs. The phenol removal was repeated over five cycles in the G-NDs/PDS system by augmenting 0.01 mM phenol and 1 mM PDS after the completion of each cycle (Figure 5). The degree of phenol removal decreased by 35% after five times recycling, which is likely due to the loss of active sites on the G-ND surfaces through the oxidation of graphitic carbons and the adsorption of phenol and phenol oxidation products (further discussed below). After the fifth cycle, suspended G-NDs in solution was treated using UV−C irradiation for 3 h (1.8 mW/cm2 from 4 W low-pressure mercury lamp, Philips Co.). Water temperature change was minimal over the course of the UV irradiation. The UV−C

Figure 5. Repeated degradation and recovery of phenol by G-NDs with persulfate systems: [phenol]0 = 0.01 mM; [G-ND]0 = 0.1 g L−1; [PDS]0 = 1 mM; 1 mM phosphate buffer at pH0 7.

treatment partially recovered the degree of phenol removal, while the efficiency decreased again by approximately 50% at the fourth cycle after the regeneration. The FT-IR analysis of G-NDs before and after the UV−C treatment (SI Figure S10) show that out-of-plane bend of aromatic C−H (730 cm−1) and symmetric and asymmetric vibrations of SOS of sulfonate group (1074 and 1171 cm−1)35 weaken after the UV−C treatment, suggesting that the phenol and/or its oxidation products and PDS were removed from the G-ND surface by the UV−C treatment. Mechanism of PDS Activation and Phenol Oxidation by G-ND. The removal of phenol by the G-NDs/PDS system was examined in the presence of oxidant scavengers (Figure 6a). Methanol and dimethyl sulfoxide were used as scavengers for SO4•− and •OH32,36−38 and L-histidine39 and azide ion40 as singlet-oxygen (1O2) scavengers. The second-order rate constants for reactions of these oxidant scavengers with SO4•−, •OH, and 1O2 are all in the order of 109 M−1 s−1.32,33 An excess amount (200 mM) compared to phenol (0.01 mM) was added to ensure complete scavenging, but we observed only partial inhibitory effects. This result indicates that SO4•−, •OH, and 1O2 are not the major oxidants responsible for the phenol degradation in the GNDs/PDS system. The observation that benzoic acid oxidation (approximately 20% over 60 min) was not accompanied by the formation of 4-hydroxybenzoic acid, a well-known product of hydroxyl radical reaction with benzoic acid, further supports that •OH is not likely involved. In addition, if SO4•− were produced, benzoic acid would have been much more rapidly degraded. For example, if we assume SO4•− as a major oxidant in this system, benzoic acid should degrade at a rate of 0.115 min−1 (half-life = 6 min) based on the fact that phenol degraded at a rate of 0.846 min−1 (the second-order rate constants for reactions of SO4•− with phenol and benzoic acid are 8.8 × 109 M−1 s−1 and 1.2 × 109 M−1 s−1, respectively). This is far from what we observed in Figure 3c. The addition of different NOMs also only partially inhibited the phenol removal (Figure 6b), which contrasts to other radical-mediated oxidation processes of which the removal rate E


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Figure 6. Phenol oxidation by G-NDs/persulfate systems in the presence of (a) reactive oxidant scavengers and (b) natural organic matter: [Phenol]0 = 0.01 mM; [G-ND]0 = 0.1 g L−1; [PDS]0 = 1 mM; [Methanol]0 = [DMSO]0 = [L-histidine]0 = [azide ion]0 = 200 mM; [Suwannee River NOM]0 = [Suwannee River Fulvic Acid]0 = [Elliott Soil Humic Acid]0 = [Suwannee River Humic Acid]0 = 10 ppm; 1 mM phosphate buffer at pH0 7.

Figure 7. (a) TGA curves, (b) FT-IR spectrum, (c) linear sweep voltammograms, (d) DOC removal by G-NDs in the presence of PDS or/and phenol: (a): ([phenol]0 = 0.1 mM; [PDS]0 = 1 mM; 1 mM phosphate buffer at pH0 7. (b), (c), (d): [Phenol]0 = 0.1 mM; [G-ND]0 = 0.1 g L−1; [PDS]0 = 1 mM; 1 mM phosphate buffer at pH0 7.

is significantly affected by radical-scavenging NOM.41−43 Finally, the EPR analysis did not show any measurable signals of •OH and SO4•− (SI Figure S11), supporting the claim that the production of radical species does not prevail in the G-ND/ PDS system. Recently, Duan et al. have observed similarly enhanced degradation of phenol by activating persulfate using annealed NDs (similarly prepared as G-NDs in this study) compared to pristine NDs.24 They proposed that •OH is likely responsible for the phenol degradation based on the EPR observation that

contradicts to our results; that is, a DMPO−OH signal was detected in their system. However, in spite of the greater reactivity of annealed NDs for persulfate activation, their EPR data did not show any significant difference in the signal intensity between annealed and pristine NDs, which raises questions about their claim. In addition, they presented the data showing that the addition of excess radical scavengers did not completely inhibit the phenol degradation (similar to the results of Figure 6), which is also not supportive of the mechanism involving •OH. F


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limit the spectrum of degradable target contaminants, it minimizes the loss of oxidation power by undesired reactions with background substances such as NOMs, as evidenced by effective phenol removal in select natural water samples (e.g., surface water, groundwater, seawater) (SI Figure S14). The GND is particularly appealing to sustainable environmental application considering its carbon-only composition (no precious metals), no known toxicity, relatively low costs (e.g., much lower than other carbon-based materials such as CNTs), and facile method for partial regeneration (e.g., the UV−C treatment as demonstrated in this study).

Alternatively, we postulate that a nonradical mechanism should be involved in the PDS activation by G-NDs and accompanying phenol oxidation. The PDS activation and phenol degradation involve electron transfer from phenol (an electron donor) to PDS (an electron acceptor), in which GND’s engagement as a facile electron transfer mediator is essential. Such an electron transfer would be facilitated as GND surface serves to bring together electron donor and acceptor pairs to a close proximity. In TGA analysis, we observed that the mass loss rate increased in the order of GNDs < G-NDs/PDS < G-NDs/PDS/Phenol (Figure 7a). This result indicates that PDS and phenol are likely to be strongly bound to G-ND surfaces and become liberated as temperature is elevated to 200−400 °C during TGA analysis. The FT-IR spectrum of G-ND/PDS (Figure 7b) shows increased absorption at around 1171 and 1074 cm−1 (believed to result from symmetric and asymmetric vibrations of SOS35 of sulfonate group) compared to that of G-ND, indicating the adsorptive interaction of PDS on G-ND surface. The absorption peak at 730 cm−1 observed in G-NDs/PDS/phenol is assigned to the out-of-plane bend of aromatic C−H originating from phenol. We further observed a significant current increase toward the G-ND electrode in the LSV analysis only when both PDS and phenol are present (Figure 7c). This observation suggests that close interaction of both PDS and phenol to G-ND surface is essential to facile electron transfer from phenol to PDS. A similar observation has been made for the CNTs/PDS system based on the LSV result using a CNT electrode. 18 Chronoamperometric measurements more clearly show this phenomenon (refer to the inset of Figure 7c). After the injection of PDS, a small negative current peak was detected due to the instant electron movement from the G-ND electrode to PDS when the PDS interacts with G-ND surface, most likely through the formation of charge transfer (electron donor−acceptor) complex. Subsequently, upon the addition of phenol, a strong positive current flow forms as electrons are transferred from phenol to the PDS/G-ND complex likely through the formation of charge transfer complex between GND and phenol. During the oxidation of phenol, we observed approximately 65% reduction in DOC within 60 min of reaction time (Figure 7d). The DOC reduction proceeded even after the complete degradation of phenol. It is interesting to note that, even though the phenol adsorption to G-ND is almost negligible in the absence of PDS (Figure 3a), a fraction of phenol can adsorb to G-ND/PDS. We also observed that, after the complete removal of phenol by the G-NDs/PDS system, treating G-NDs with base at an elevated temperature recovered a portion of phenol (approximately 10%) (SI Figure S12). These results suggests that DOC removal is likely due to adsorption of phenol and phenol oxidation products (primarily p-benzoquinone according to GC/MS analysis (SI Figure S13)) to G-ND surface in the presence of PDS. Environmental Implications. This study demonstrates that PDS/PMS activated by G-ND can be applied as an alternative oxidation process for the removal of organic contaminants in water. Different from conventional activatedpersulfate systems that generate reactive radical species, the GND/persulfate system exhibits relatively selective reactivity toward specific organic compounds including phenols as the oxidation involves the formation of charge transfer complex between PDS/phenol and G-ND. While such selectivity may

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b02079. Detailed experimental setup and procedure information on GC/MS system for phenol oxidation products analysis (Text S1), water quality parameters of field water samples (Table S1), phenol removal by G-NDs/ PDS system with and without phosphate buffer solution (Figure S1), phenol removal by G-NDs/PDS system with and without dissolved oxygen (Figure S2), photographs and XPS spectra of G-NDs prepared at different annealing temperatures (Figure S3), HR-TEM images of G-ND (Figure S4), particles size distribution and average particles size for aqueous suspension of G-ND measured by dynamic light scattering (Figure S5), phenol oxidation by surface modification nanodiamonds without persulfate systems (Figure S6), phenol removal and pseudo first order rate constant for the removal of phenol by G-NDs/ PDS system as a function of G-NDs loading and phenol concentrations (Figure S7), decomposition of phenol and oxyanions by G-NDs in the presence of oxyanions (Figure S8), BET specific surface area of carbonmaterials (Figure S9), FT-IR spectrum of G-NDs/PDS system with and without UV treatment (Figure S10), EPR spectra obtained by spin trapping with DMPO in the Fe(II)/H2O2 and G-NDs/PDS system at acidic pH and neutral pH (Figure S11), the GC/MS spectrum of intermediates for the removal of phenol by G-NDs/PDS system (Figure S12), removal and recovery of phenol by G-NDs with and without persulfate systems (Figure S13), phenol removal by G-NDs/PDS system with different type of tap water (Figure S14) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(C.L.) Phone: +82-52-217-2812; fax: +82-52-217-2809; email: [email protected]. *(J.-H.K.) Phone: +1-203-432-4386; fax: +1-203-432-4387; email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work was supported by Korea Institute of Toxicology and by the National Research Foundation of Korea (NRF) grants funded by the Korean government (MSIP) (NRF2015R1A5A7037825; NRF-2015R1A2A1A15055840). G


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